Glass materials for optical gain media and infrared optics comprising rare earth oxide glass compositions

ABSTRACT

This invention relates to the use of novel glass materials comprising rare earth aluminate glasses (REAl™ glasses) in the gain medium of solid state laser devices that produce light at infrared wavelengths, typically in the range 1000 to 3000 nm and for infrared optics with transmission to approximately 5000 nm in thin sections. The novel glass materials provide stable hosts for trivalent ytterbium (Yb 3+ ) ions and other optically active species or combinations of optically active species that exhibit fluorescence and that can be optically excited by the application of light. The glass gain medium can be configured as a waveguide or placed in an external laser cavity, or otherwise arranged to achieve gain in the laser waveband and so produce laser action.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under contract numberDMI-0216324 awarded by the National Science Foundation and contractnumber F49620-02-C-0028 awarded by the Air Force Office of ScientificResearch. The Government has certain rights in this invention.

FIELD OF INVENTION

This invention relates to solid state lasers that use novel glasscompositions, comprising rare earth oxides and aluminum oxide (the REAl™glasses) doped with optically active species, as the gain medium. Itfurther relates to lasers based on these glass compositions that emitinfrared light in the wavelength range from approximately 1000 to 3000nm through the application of pump radiation at a wavelength of 970 nmto 990 nm, and preferably about 980 nm. It further relates to the use ofREAl™ glasses that can be cast in the form of “blanks” that formcomponents of laser gain media and windows, filters, or lenses thattransmit infrared light.

BACKGROUND OF INVENTION AND DESCRIPTION OF THE PRIOR ART

The composition range of the REAl™ glasses is stated in U.S. Pat. No.6,482,758, Nov. 19, 2002 incorporated herein by reference.

Glass materials are generally manufactured by starting with a liquid,formed by melting solid crystalline starting materials. The liquid iscooled in a way that prevents crystallization. While there are otherways to make glass, forming it from the liquid provides a simple way toachieve large pieces of material that can readily be formed intoproducts. Here we show that by virtue of their optical, mechanical andthermal properties and the ability to fabricate the glasses by castingfrom a liquid, the REAl™ glasses provide a novel material for the gainmedium used to construct infrared laser devices and for optical elementssuch as windows and lenses.

It should be noted that certain fabrication, coating, and otheroperations that are well-known in the art are typically employed toprepare components of devices from the glass optical materials andoptical gain media of this invention.

Lasers that produce infrared light (“infrared lasers”) are widely usedin materials processing, optical communications, medical and dentaldiagnostics and surgical procedures, optical range finding and remotesensing, and numerous applications in analysis, marking, scribing,engraving and optical diagnostics. High power density lasers thatprovide a quality beam profile at infrared wavelengths are useful inmaterials processing operations including welding, metal cutting andmetal forming operations, and medical procedures. Infrared lasers arealso used in military applications for range finding, targetdesignation, and missile guidance systems. Infrared lasers also haveapplication in Homeland security, where sensors, laser-based detection,and laser-based defense systems that employ infrared lasers and lasertechnologies are being developed.

Many solid state lasers, for example the “neodymium:YAG” laser, employtrivalent rare earth ions distributed in a medium such as a crystal or aglass material that can be “pumped” to excite the laser active ions.Neodymium, erbium and ytterbium are widely used to generate light atinfrared wavelengths. The gain medium provides a host for the laseractive ions and forms a critical component of the laser. The gain mediummust be able to transmit light at the laser wavelength with minimallosses. It may also provide a means to extract heat generated by theoptical processes, and in some instances it provides a structuralelement of the laser itself. The gain medium may also be formed as thelaser cavity by placing reflective coatings on various surfaces. Solidstate lasers that employ a REAl™ glass doped with optically activespecies are within the scope of this invention.

The advent of high power density lasers based on Yb-doped YttriumAluminum Garnet (YAG) crystals containing several percent ytterbium hasshown the utility of Yb lasers that can be pumped over a narrowwavelength range by using commercially available infrared laser diodes.Ytterbium ions are a desirable dopant for laser applications because,unlike other optically active rare earth ions, electronically excited Ybions do not suffer from energy-sapping cross relaxation andexcited-state absorption processes. Pumping the strongly absorbing²F_(7/2) state in trivalent Yb ions with laser diodes overcomes thelimitation of low pump absorption with the broadband lamp pumpingschemes commonly used in Nd-based lasers. The close spacing of theabsorption and emission bands in Yb³⁺ results in small conversionlosses.

While the Yb lasers were first demonstrated as flashlamp-pumped devicesin 1965, it is only recently that these lasers have acquiredtechnological significance, through advances in pump sources, laser gainmedia, and laser output power that can be achieved. Small, diode-pumpedYb-doped rod lasers were first demonstrated at the Lincoln Laboratoryaround 1990. Subsequent laser development at Lawrence Livermore NationalLaboratory, Raytheon and other laboratories in the US and abroad hasincreased the power output of small (˜5 mm diameter, 10 mm length) rodlasers towards 1 kW to provide an enormous specific power. The thin diskYb:YAG laser was pioneered in Germany. Power output of ˜650 Watts hasbeen demonstrated in 0.2 mm thickness disks pumped in a region a fewmillimeters in diameter. The disk laser is predicted to enable a poweroutput of ca. 10 kW from a single small disk laser device. By providinga larger planar surface for heat extraction than is possible in a longcylinder, the disk laser has potential to achieve the maximum possiblepower density. The wide availability of inexpensive and electricallyefficient InGaAs-based laser diodes which operate in the 940-980 nm pumpwavelength range needed to realize Yb-based lasers has laid thefoundation for new near IR power laser products. Optical efficiencies ofaround 50 % are achieved in disk laser configurations operating nearroom temperature; even higher efficiencies have been obtained usingcryogenically cooled disks.

The present invention provides novel glass host materials for the Ybions, i.e., the “REAl™” glasses comprised of rare earth oxides andaluminum oxide, that are used to make Yb: REAl™ glass laser devices.Technical drawbacks of crystalline Yb:YAG lasers relative to the lasersof the present invention are: (i) the Yb³⁺ absorption band typicallynecessitates pumping at around 940 nm, rather than 980 nm whereinexpensive and powerful diode laser pump sources are available, (ii)pumping at 940 nm rather than 980 nm, in combination with laser emissionat a wavelength of ˜1030 run, leads to increased heat generation whichlimits the total power density that can be achieved, (iii) the smallermagnitude of the ground state absorption in Yb:YAG, reduces theefficiency of pump power utilization, and (iv) strain-inducedbirefringence in melt grown crystals due to growth stresses and latticestrain can produce beam deflection and instability in the laser cavity.

Lasers and devices that transmit infrared radiation that are based onREAl™ glasses also have potential cost advantages over the YAG- andother crystalline host-based devices because the glass formingoperations are relatively inexpensive compared with crystal growingoperations.

The use of REAl™ glasses for windows, lenses, filters, and other opticalapplications that require infrared transmitting material benefits from(i) the large Abbe number, (ii) the range of Abbe numbers, and (iii) theIR transmission to wavelengths of ˜5000 nm, and (iv) the largerefractive index of these materials. The REAl™ glasses provide superiorvalues of these properties relative to the familiar silicate glasses.The REAl™ glasses also provide thermal, chemical, and environmentalstability that is superior to other infrared transmitting materials suchas fluoride and tellurite glasses.

SUMMARY OF THE INVENTION

The invention is an optical gain medium comprising a bulk single phaseglass. The bulk single phase glass comprises 27 to 50 molar % RE203 and50 to 73 molar % Al₂O₃, where RE is one or more elements selected fromthe group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm, Yb and Lu. The optical gain medium may be used in a mannersuch that gain is generated by application of light in the wavelengthrange from 970-990 nm. The optical gain medium may be doped withytterbium ions or other dopant ions such as Er, Tm or Ho. Gain may begenerated by electronic transitions of Yb or other dopant ions such asEr, Tm or Ho.

In a second aspect of the invention, the invention is an optical gainmedium consisting essentially of a bulk single phase glass comprisingone or more rare earth oxides, aluminum oxide and silicon dioxidewherein the composition of the bulk single phase glass liessubstantially within the heptagonal region of the ternary compositiondiagram of the rare earth oxide-alumina-silica system defined by pointshaving the following molar percent compositions: 1% RE₂O₃, 59% Al₂O₃ and40% SiO₂; 1% RE₂O₃, 71% Al₂O₃ and 28% SiO₂; 23% RE₂O₃ and 77% Al₂O₃; 50%RE₂O₃ and 50% Al₂O₃; 50% RE₂O₃ and 50% SiO₂; 33.3% RE₂O₃, 33.33% Al₂O₃and 33.33% SiO₂; and 16.67% RE₂O₃, 50% Al₂O₃ and 33.33% SiO₂. Theoptical gain medium may be used in a manner such that gain is generatedby application of light in the wavelength range from 970-990 nm. Theoptical gain medium may be doped with ytterbium ions or other ions suchas Er, Tm or Ho. Gain may be generated by electronic transitions of Yb,Er, Tm of Ho.

In a third aspect of the invention, the invention is an optical materialconsisting essentially of a bulk single phase glass comprising 27 to 50molar % RE₂O₃ and 50 to 73 molar % Al₂O₃, where RE is one or moreelements selected from the group consisting of Sc, Y, La, Ce, Pr, Nd,Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu and wherein the glass isformed by casting of a molten material.

In a fourth aspect of the invention, the invention is an opticalmaterial consisting essentially of a bulk single phase glass comprisingone or more rare earth oxides, aluminum oxide and silicon dioxidewherein the composition lies substantially within the heptagonal regionof the ternary composition diagram of the rare earthoxide-alumina-silica system defined by points having the following molarpercent compositions: 1% RE203, 59% Al₂O₃ and 40% SiO₂; 1% RE₂O₃, 71%Al₂O₃ and 28% SiO₂; 23% RE₂O₃ and 77% Al₂O₃; 50% RE₂O₃ and 50% Al₂O₃;50% RE₂O₃ and 50% SiO₂; 33.33% RE₂O₃, 33.33% Al₂O₃ and 33.3% SiO₂; and16.67% RE₂O₃, 50% Al₂O₃ and 33.33% SiO₂, where RE is one or moreelements selected from the group consisting of Sc, Y, La, Ce, Pr, Nd,Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu and wherein the glass isformed by casting of a molten material.

DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram illustrating the use of REAl™ glass asgain medium in a solid state laser.

FIG. 2 illustrates the absorption cross section spectrum of a REAl™glass containing Yb³⁺ ions.

FIG. 3 shows the emission spectrum of a REAl™ glass doped with Yb³⁺ ionsand excited by a 980 nm diode pump laser.

FIG. 4 shows the fluorescence decay curve observed in a REAl™ glassdoped with Yb³⁺ ions, giving a fluorescence lifetime of Yb³⁺ ions ofapproximately 800 microseconds.

FIG. 5 illustrates change in fluorescence lifetime of Yb³⁺ ions in REAl™glass with changes in the Yb³⁺ concentration and with the SiO₂ contentof the glass.

FIG. 6 shows the fluorescence decay curves observed in a REAl™ glassdoped with Er³⁺ ions and with another REAl™ glass that was co-doped withEr³⁺ and Yb³⁺ ions.

FIG. 7 shows the emission spectrum of a REAl™ glass doped with Er³⁺ andTm³⁺ ions and excited by a 980 nm diode pump laser.

FIG. 8 shows the emission spectrum of a REAl™ glass doped with Er³⁺ andHo³⁺ ions and excited by a 980 nm diode pump laser.

FIG. 9 shows fluorescence decay curves for the emission of infraredradiation at wavelengths of approximately 1550 nm and approximately 3000nm from Er-doped crystalline YAG and from Er-doped REAl™ glass.

FIG. 10 shows the infrared transmission as a function of wavelength for2 mm thick samples of REAl™ glasses containing zero to 20 mole % SiO₂,pure silica, and single crystal sapphire.

DETAILED DESCRIPTION OF INVENTION

This invention relates to the use of the REAl™ glass materials dopedwith up to 20 mole % Yb₂O₃ as the gain medium in solid state infraredlasers. The invention further relates to REAl™ glass gain mediacontaining additional optically active rare earth ions that may beoptically excited by energy transfer from excited ytterbium ions, e.g.Er³⁺, Tm³⁺, Ho³⁺, and combinations thereof. By combing ytterbium and theadditional optically active ions, the high efficiency of pump absorptionat 980 nm by Yb³⁺ can be exploited to provide a reservoir of energy toexcite the additional dopants by energy transfer from the excitedytterbium ions.

The REAl™ glass materials are based on rare earth oxide and aluminumoxide, and may comprise up to 30 mole % of SiO₂. In this disclosure, weshow that these glasses have properties favorable to operation of novellaser devices and that they maintain these properties at the high dopantconcentrations that are possible in the REAl™ glass family of materials.The glass materials have a wide homogeneity range so that the dopantconcentrations are not restricted by stoichiometric considerations thatmay limit the concentrations of dopants in crystalline hosts. Further,unlike glass materials, high dopant concentrations tend to producebirefringence and strain in crystalline materials. The glasses can becast into a variety of forms by melting starting materials in a platinumcrucible. Some of the compositions have melting temperatures that exceedthe approximately 1950K upper temperature limit for processing inplatinum crucible. These higher-melting compositions may be cast intoglass after melting in an iridium crucible. While casting is known inthe art of glass making, its application in REAl™ glass synthesis isnovel. Prior art syntheses of REAl™ glasses have employed high coolingrates to form the glasses. The prior art cooling rates exceed thoseachieved in the casting operations, and it has not been previouslydemonstrated that synthesis of bulk REAl™ glasses by casting operationsused in the present invention is possible. Previously, the REAl™ glasseswere synthesized using levitation melting techniques that avoidednucleation of crystals in the liquid. The new glasses can be cast toform rods, plates and a wide variety of shapes. These products may befinished if necessary, by polishing, machining, or other conventionaloperations, to form the laser gain block components, windows, andoptical components such as lenses or filters that exploit absorptionbands of optically active dopant ions. Tables I and II presentcompositions of REAl™ glasses that can be formed by casting fromplatinum or iridium crucibles. TABLE I Examples of glass compositions.Balance is Al₂O₃ in all cases. Chemical Composition, Mole PercentExample Y₂O₃ La₂O₃ Other Oxides I-A 10 20   20 SiO₂ I-B 25   30 SiO₂ I-C20   30 SiO₂ I-D 20   25 SiO₂ I-E 10 15   25 SiO₂ I-F 10 10   30 SiO₂I-G 7.5 15 2.5 Gd₂O₃   20 SiO₂ I-H 9 15   1 Gd₂O₃   10 SiO₂ I-I 7.5 152.5 Gd₂O₃   15 SiO₂ I-J 7.5 15 2.5 Gd₂O₃   20 SiO₂ I-K 7.5 15 2.5 Gd₂O₃  15 SiO₂ I-L 5 15   2 Gd₂O₃   2 ZrO₂   10 SiO₂   2 Sc₂O₃   2 HfO₂   2Lu₂O₃ I-M 7 13.5   2 Gd₂O₃ 22.5 SiO₂ I-N 7.5 12 0.5 Gd₂O₃   15 SiO₂ I-O7.5 15 2.5 Gd₂O₃   18 SiO₂ I-P 5.8 6.5 14.8 ZrO₂ 21.1 SiO₂

TABLE II Examples of glass compositions that contain optically activedopants. Balance is Al₂O₃ in all cases. Chemical Composition, MolePercent Example Y₂O₃ La₂O₃ Other Oxides II-A 14.6 0.4 Er₂O₃ 30 SiO₂ II-B19   1 Er₂O₃ 25 SiO₂ II-C 5 15   5 Er₂O₃ 20 SiO₂ II-D 5 15   5 Nd₂O₃ 20SiO₂ II-E 7.5 10 2.5 Gd₂O₃   5 Nd₂O₃ 20 SiO₂ II-F 5 15   2 Gd₂O₃   2Er₂O₃ 10 SiO₂   2 ZrO₂   2 Ho₂O₃   2 HfO₂ II-G 5.5 15 2.5 Gd₂O₃   2Yb₂O₃ 20 SiO₂ II-H 7 15   2 Gd₂O₃   2 Yb₂O₃ 20 SiO₂   2 ZrO₂   2 HfO₂II-I 4.5 15 2.5 Gd₂O₃   3 Yb₂O₃ 20 SiO₂ II-J 3.5 15 2.5 Gd₂O₃   4 Yb₂O₃20 SiO₂ II-K 9 18   3 Yb₂O₃ 15 SiO₂ II-L 7 15   3 Yb₂O₃ 20 SiO₂ II-M 715   2 Gd₂O₃   5 Yb₂O₃ 15 SiO₂   1 Er₂O₃ II-N 7 17   2 Gd₂O₃   3 Yb₂O₃15 SiO₂   1 Er₂O₃ II-O 7.5 15 2.5 Gd₂O₃   3 Er₂O₃ 20 SiO₂   1 Tm₂O₃ II-P7.5 15 2.5 Gd₂O₃   3 Er₂O₃ 20 SiO₂   1 Ho₂O₃ II-Q 7.5 15 2.5 Gd₂O₃   3Er₂O₃ 20 SiO₂   1 Dy₂O₃

When they are doped with ytterbium the glasses provide a high groundstate absorption cross section for Yb³⁺ ions that is approximately 2.5times larger than for crystalline YAG. The Yb-dopant is added in thisinstance via ytterbium oxide Yb₂O₃. The Yb may be added by use ofpotentially any source or combination of sources of trivalent ytterbiumsuch as a carbonate, oxalate, oxide, or other forms.

The ground state absorption cross section of ytterbium ions is shown asa function of wavelength for a Yb-doped REAl™ glass in FIG. 2. The peakabsorption is closely matched to the 980 nm laser diode wavelength whichenables the use of inexpensive diode lasers for pumping. Thefluorescence emission spectrum of the ytterbium ions is shown in FIG. 3.In this figure, the off-scale peak at ˜980 nm is due to diode laser pumplight used to excite the fluorescence. The small separation inwavelength between the pump and the Yb laser emission, which typicallyoccurs at ˜1030 nm, means that the Yb:REAl™ glass laser can be moreefficient than the Yb:YAG crystal devices. In particular, use of thelonger wavelength 980 nm pump radiation in Yb:REAl™ glass will reduceheat generation in the gain medium. Heat in the gain medium results inchanges in density and optical properties, wavefront distortion andultimately limits the power that can be extracted from a device. The useof the new glass materials of this invention provides the basis formore-efficient lasers that employ gain media formed by glass castingoperations that are inexpensive compared with the crystal growthoperations required to make Yb:YAG lasers.

As shown in U.S. Pat. No. 6,438,152, glasses have been made with up to20 mole % Yb₂O₃ and with mixtures of Yb₂O₃ and other optically activedopants such as Er₂O₃. As described in the prior art, these glassesprovide a high solubility of all the rare earths. A wide range of rareearth dopant compositions can be used, thus energy transfer processesbetween different rare earth ions can be exploited as a means to obtainhigh pump utilization efficiency. In addition, codoping with Yb andother rare earth ions enables the use of 980 nm laser diodes to excitelaser action from species that do not absorb the 980 nm pump radiation.

A further property of ytterbium ions in the REAl™ glass that makes ituseful in laser devices is the fluorescence lifetime of excited Yb³⁺ions. A measurement of the fluorescence lifetime of excited Yb³⁺ ions inREAl™ glass is shown in FIG. 4. A plot of the fluorescence lifetime ofexcited Yb³⁺ ions in REAl™ glasses is shown as a function of Ybconcentration in FIG. 5. The lifetime is comparable to the Yb³⁺fluorescence lifetime in other hosts, i.e., 0.5 to 1 ms.

In addition to the advantageous spectroscopic properties of Yb-dopedREAl™ glass, the materials can be formed using relatively low costprocesses compared to those required to fabricate single crystalmaterials. The glasses can be cast in various forms by pouring moltenmaterial into molds. The molds can be maintained at an elevatedtemperature and allowed to cool slowly after the glass is formed torelieve stress in the as-formed glass. The glass may also be cast into amold that is initially at room temperature. The glasses can be annealedat temperatures up to ˜1100K to relieve stresses. The addition of rareearth ions does not result in lattice strains in the amorphous hosts.The glasses are homogeneous. The use of Yb-doped REAl™ glass thusenables lasers with the following properties:

-   -   High optical conversion efficiency    -   High laser power output    -   Minimal operating temperature at given laser power output    -   Wide range of compositions not restricted by crystal        stoichiometry    -   Easy fabrication of the gain medium    -   Optically isotropic gain medium    -   Efficient absorption of pump radiation    -   Robust and compact devices

Table III presents properties of the REAl™ glass materials that havebeen measured on samples of materials formed either by levitationmelting and cooling or by casting liquids formed in platinum crucibles.TABLE III Properties of REAl ™ glass materials Property Range of valuesMajor components Al₂O₃, RE₂O₃*, 0-35 mole % SiO₂ Solubility of rareearth oxides Up to 50 mole % RE₂O₃ Spectral transmission range Near UVto ˜5500 nm Refractive index (n_(D), λ = 589 nm) 1.7 to >1.8 Abbe number(n_(D) − 1)/(n_(F) − n_(C)) 40-60 Hardness 800-1000 VickersDevitrification temperature 950-1050° C. Thermal conductivity 0.01 W/cm· ° C. (at 20° C.) Thermal expansion coefficient ˜10 × 10⁻⁶/° C. Density3.4-4.1 gram per cm³ Young's modulus 110-130 GPa (16 MSI) Chemicalstability (in water Dissolution rate <1 × 10⁻⁸ g/cm²/min at 90° C.)*Oxides of elements: Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu.

PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 is a schematic diagram of a solid state laser device thatincorporates a doped REAl™ glass and illustrates the preferredembodiment of the invention. Optical radiation 1 that excites the lasermedium is provided by a pump light source 2 and directed to the lasergain medium 3. Mirrors 4 are located at opposite ends of the laser gainmedium, one of which is partially transmitting to yield the laser output5. Cooling means 6 may be incorporated to achieve increased laser poweroutput from the device.

The pump light source 2 is preferably a 980 nm laser diode light sourcebut it may be any light source capable of exciting the optically activeions in the gain medium 3. The gain medium 3 is a REAl™ glass ofcomposition within the phase field stated in U.S. Pat. No. 6,482,758,preferably a composition that contains approximately 10 mole % SiO₂ thatcan be melted in a platinum crucible and formed into a glass byconventional casting methods known in the art of glass making. The gainmedium 3 is doped with optically active species, preferably rare earthions such as Yb³⁺, Er³⁺, Tm³⁺, Ho³⁺, or combinations thereof. Any otherdopant species capable of producing laser emission from a REAl™ glass,including other optically active rare earth ions may also be used. Thepartially reflecting output mirror is preferably constructed from REAl™glass that is not doped with optically active species but it may be ofany glass or crystalline material that exhibits high transmission at thewavelength of the laser radiation. Other components of the device areknown in the prior art of lasers and optical devices. For example, thesurfaces of the gain medium may be coated to reduce reflections.

EXAMPLE 1 Cast REAl™ Glasses

The cast REAl™ glasses were prepared from mixtures of fine powders ofthe constituent pure oxides. The oxides were first melted together in alaser hearth. The product of hearth melting was then pulverized, placedin a platinum crucible, and heated in a Deltech DT31FL high temperaturefurnace to a temperature of 1920 to 1950K to obtain a homogeneous moltenoxide. The platinum crucible was then removed from the furnace and theliquid oxide was cast into a mold to produce the glass products. In somecases the mold was heated to allow in-situ stress relaxation of theas-cast glass by slowly cooling the mold. In other cases the glass wascast into a mold at room temperature and could later be annealed attemperatures up to approximately 1100K. Graphite molds were used for thecasting operations. Other mold materials that are commonly used in theart of glass making are within the scope of this invention.

The process of hearth melting and pulverization of the hearth-meltedproduct are not essential steps in the glass synthesis. They were usedfor convenience in the laboratory synthesis work, to (i) homogenize thematerials, (ii) minimized the time at temperature required in theplatinum crucible melting step, and to (iii) increase the density of thematerial placed in small platinum crucibles, and (iv) facilitatereaction of the high melting components to ensure complete melting atthe process temperature for crucible melting.

Tables I and II list compositions that were cast into glasses andcompositions for which the glass was obtained directly from the laserhearth melting operation. In all cases, a glass was obtained. Somecrystalline material was often observed at the surface of the glasswhich, along with any glass whose composition is influenced by thecrystallization, could be removed by grinding and polishing operations.Melting in a crucible, such as an iridium crucible, whose melting pointexceeds that of pure platinum may be employed to cast glasses such asthe REAl™ glasses containing less than approximately 5 mole % Sio₂ whosemelting point exceeds the melting point of platinum.

It is known in the art that various starting materials may be used toobtain the final compositions of the REAl™ glasses. For example, solgels may be used to achieve an intimate mixture of the glass componentswhich will yield pure oxide liquid when heated and melted in air oroxygen. Carbonates and/or hydroxides may be used as starting materials,which will decompose to oxides, by the evolution of carbon dioxide orwater vapor, respectively, when heated. Also, mixed rare earth oxidesmay be substituted for the pure oxides used in the present glasssyntheses.

EXAMPLE 2 Glasses for Optical Property Investigations

Several hours are required to complete the procedure of casting a REAl™glass from a crucible. Small glass samples that are sufficient foroptical property investigations can be prepared in a few minutes, bycontainerless melting techniques. Therefore, many of the compositions ofglass that were used to investigate the optical properties of REAl™glasses as a function of glass composition were prepared by thecontainerless melting methods.

EXAMPLE 3 Yb Optical Properties in REAl™ Glass

FIGS. 2-5 illustrate various optical properties of Yb³⁺ ions in REAl™glass. The ground state absorption spectrum of Yb³⁺ is shown in FIG. 2.The peak absorption cross section is approximately 2×10⁻²⁰ cm², at awavelength of 980 nm. The absorption peak is quite narrow in acrystalline host material, such as the yttrium aluminum garnet crystalsthat are used in prior art Yb:YAG lasers. The absorption peak isbroadened in a glass material, which facilitates laser pumping byincreasing the pump laser waveband that can be used. Thus, Yb:YAG laserstypically use a pump laser operating at approximately 940 nm where arelatively narrow absorption peak occurs, with a much smaller absorptioncross section than at the 980 nm peak in REAl™ glasses. The broadened980 nm absorption peak of Yb³⁺ ions in a REAl™ glass host have thebenefits, relative to prior art Yb:YAG lasers, including that (i) moreefficient laser pumping is possible, (ii) inexpensive and powerful diodelasers are available for operation at the 980 nm pump wavelength, and(iii) the typical Yb³⁺ laser wavelength is approximately 1030 nm, anduse of the 980 nm pump wavelength in Yb:REAl™ glass reduces heating ofthe gain medium.

The emission spectrum of Yb³⁺ in REAl™ glass is shown in FIG. 3. Thisspectrum was observed by exciting a sphere of the glass with the focused980 nm diode laser beam and measuring the Yb³⁺ fluorescence emission atan angle of 90° to the incident pump laser beam. Some of the pumpradiation was internally reflected at the glass surface and scatteredinto the spectrometer, to give the off-scale peak at 980 nm in theemission spectrum. The emission spectrum shows strong emission in theapproximately 1030 nm range that is typical of Yb³⁺ lasers.

FIG. 4 illustrates a measurement of the Yb³⁺ fluorescence decay rate. Inthis experiment, a disk of Yb-doped REAl™ glass, approximately 2 mmthick, was excited by the 980 nm pump laser and the emission decay wasmeasured with an InGaAs detector when the pump laser was turned off. Thepump laser path was co-linear with the axis on which fluorescence wasmeasured. A single crystal silicon disk was placed in this path, toabsorb the 980 run pump laser radiation while passing thelonger-wavelength tail of the Yb³⁺ fluorescence emission. The siliconfilter avoided saturation of the detector with-pump light so thatrecovery from saturation would not limit measurements immediately whenpumping was terminated. Pumping was terminated precisely at the pointwhere the constant intensity begins to decrease, at approximately 70 msas read on the horizontal axis of the plot. This experiment wasperformed on a glass whose composition, in mole %, was 4 Yb₂O₃, 62.5Al₂O₃, and 33.5 La₂O₃. It can be seen that decay of the fluorescencesignal is precisely exponential, i.e., a plot of the logarithm ofintensity versus time is linear, with a slope corresponding to a decaytime constant of approximately 0.80 ms.

FIG. 5 plots the fluorescence decay times for several Yb concentrations.The top part of this figure shows results for a glass free from SiO₂ andthe bottom part of the figure shows results for a glasses containing 2mole % Yb₂O₃ and up to 20 mole % of SiO₂. It can be seen that thefluorescence decay rates decrease with the SiO₂ content of REAl™ glass,and are typical of the 0.5 to 1.0 ms decay time constants observed inother Yb-doped host materials.

Larger lifetimes for the excited state facilitate storage of excitedstate energy and are generally advantageous to laser design. The resultsshown in FIG. 5 show that it is advantageous to minimize the SiO₂content of the glass host material, to achieve longer lifetimes for theexcited Yb³⁺ ions. The increased lifetime of Yb³⁺ in low-SiO₂ REAl™glasses, relative to high-SiO₂ glasses, is a novel and useful propertyachieved in this invention. The invention provides bulk REAl™ glasseswith only ˜10 mole % SiO₂ that can be melted and cast into bulk glassfrom platinum crucibles by conventional glass-making methods.

EXAMPLE 4 Co-Doped Materials

Co-doped REAl™ glass allows novel laser devices to be constructed basedon the strong pump laser absorption property of Yb³⁺ ions and the energytransfer processes that occur between the Yb³⁺ ions and co-dopedoptically active species. The ability of REAl™ glass to maintainfavorable optical properties such as large emission lifetimes with largedopant concentrations enables these devices because relatively largedopant concentrations are required to achieve rapid energy transferbetween the optically active species. The glasses that comprise this setof materials include all of the single phase glasses lying in the phasefield defined in U.S. Pat. No. 6,482,758. The dopants include, but arenot limited to, optically active rare earth elements, such as thetrivalent ions of Yb, Er, Tm, Ho, Dy, Nd, and Pr.

The fluorescence decay measurements described in the remainder of thisexample were, except as noted, performed in the same manner as the Yb³⁺fluorescence decay measurements described in example 3.

REAl™ Glass Doped with Er and Yb

FIG. 6 illustrates the fluorescence decay curves of two REAl™ glasssamples doped with Er³⁺ ions and pumped with a 980 nm diode pump laser.Emission from the excited Er³⁺ ions occurs in the well-known waveband of1500 to 1600 nm that is used in Er-doped optical communications devices.Each of the REAl™ glasses for which data are given is doped with 1 mole% Er₂O₃. The figure at the top shows results for a glass that alsocontains 2 mole % Yb₂O₃. The results in FIG. 6 illustrate the following:First, the beginning of the decay curves shows a small and suddendecrease of intensity when the pump laser is turned off, atapproximately 118 ms and approximately 52 ms on the horizontal axes ofthe top and bottom figures, respectively. This sudden decrease ofintensity is due to the termination of the pump laser light, a smallfraction of which is transmitted to the detector. This decrease issmaller for the Yb-doped sample because this sample transmits a smallerfraction of the incident pump light. Second, the Er³⁺ emission intensityis greater for the Er/Yb co-doped glass than for glass doped only withEr. This result is also due to the increased absorption that occurs inYb, which increases the level of excitation in the glass. It alsodemonstrates that transfer of excited state energy from the Yb³⁺ ions tothe Er³⁺ ions is efficient; a substantial part of the pump energyabsorbed by the Yb³⁺ ions appears as emission from Er³⁺ ions. Third, thelarge decay lifetimes observed in both sets of data, 5.9 ms for theEr-doped REAl™ glass and 6.6 ms for the co-doped glass shows that theobserved emission must be from the Er ions, since emission from Yb ionshas a much smaller lifetime of approximately 0.8 ms. The maximumpossible Yb³⁺ emission that could be detected from the co-doped sampleis much smaller than the observed emission intensity because the siliconfilter greatly reduces the Yb³⁺ intensity and has only a small influenceon the longer wavelength Er³⁺ intensity. Thus, it is not known if theco-doped glass produced significant direct emission from the excitedYb³⁺ ions.

REAl™ Glass Doped with Er and Tm

FIG. 7 illustrates the emission spectrum from REAl™ glass containing 3mole % Er₂O₃ and 1 mole % Tm₂O₃, and pumped with a 980 nm diode laser.The spectrum shows relatively weak emission from Er³⁺, in the 1500-1600nm waveband, and strong emission from Tm³⁺ in the wavelength range from1450-2000 nm. The spectrum was measured with an extended InGaAs detectorwith good sensitivity at wavelengths to more than 2050 nm. Since Tm³⁺does not absorb the pump radiation, the results given show efficientenergy transfer from the excited Er ions that are produced by absorptionof the pump light to the emitting Tm ions. This result shows that lasersand optical devices can exploit optical gain in REAl™ glass based onemission from Tm³⁺ ions, while using absorption of pump laser radiationat 980 nm, which would not be possible in a glass doped only with Tm.Since excited Yb³⁺ ions transfer energy to Er³⁺ ions in REAl™ glass, itis also possible to build similar devices with REAl™ glass doped withYb, Er, and Tm. Spectra similar to that shown in FIG. 7 were obtainedfor REAl™ glass compositions containing zero and 20 mole % of SiO₂.

REAl™ Glass Doped with Er and Ho

FIG. 8 illustrates the emission spectrum from REAl™ glass containing 3mole % Er₂O₃ and 1 mole % Ho₂O₃, and pumped with a 980 nm diode laser.The spectrum shows relatively weak emission from Er³⁺, in the 1500-1600nm waveband, and stronger emission from Ho³⁺ in the wavelength rangefrom 1800-2050 nm. The spectrum extends only to approximately 2050 nm,which was the limit for the monochromator used in the experiments. SinceHo³⁺ does not absorb the pump radiation, the results given showefficient energy transfer from the excited Er ions that are produced byabsorption of pump light to the emitting Ho ions. This result shows thefeasibility of lasers and optical devices that exploit optical gainbased on emission from Ho ions, while using absorption of pump laserradiation at 980 nm, which would not be possible in a glass doped onlywith Ho. Since excited Yb³⁺ ions transfer energy to Er³⁺ ions in REAl™glass, it is also possible to build similar devices with REAl™ glassdoped with Yb, Er, and Ho. Spectra similar to that shown in FIG. 8 wereobtained for REAl™ glass compositions containing zero and 20 mole % ofSiO₂.

EXAMPLE 5 Er Emission at a Wavelength of ˜3000 nm

Emission of infrared radiation from Er-doped REAl™ glass can be observedat a wavelength of approximately 3000 nm, in addition to the emission inthe 1550 nm waveband. FIG. 9 illustrates the decay of fluorescenceintensity for both of these emission wavelengths. The emission at ˜3000nm was measured with a mercury cadmium telluride detector in combinationwith an interference filter that transmitted light in the wavelengthrange from 2690-3190 nm. An interference filter was also used toeliminate pump laser transmission to the detector for measurements inthe 1550 nm waveband. The results in the top panel of the figure are fora YAG crystal doped with 2 mole % Er. The bottom panel shows results fora REAI™ glass doped with 3 mole % Er. In both cases, the emission at1550 nm is plotted on the left-hand scale and shows a nearly lineardecrease of log(intensity) with time. The emission at ˜3000 nm isplotted on the right hand scale. The time bases have been adjusted sothat the fluorescence decay curves begin at the same point on the timeaxes, i.e., at zero ms.

The results given in FIG. 9 show several qualities of the 3000 nmemission from Er-doped materials. First, there is an initial fast decayof the ˜3000 nm emission, which occurs from the ⁴I_(11/2) excited stateof Er³⁺. This excited state is formed by two processes: directabsorption of the pump laser radiation and cooperative upconversion ofthe lower, ⁴I_(13/2) Er³⁺ excited state. The initial decay is fromradiative loss by emission of the ˜3000 nm radiation and by quenching ofthe ⁴I_(11/2) Er³⁺ ions to form ⁴I_(13/2) Er³⁺ ions. Second, after theinitial fast decay, the 3000 nm emission exhibits slower decay. On thescales used in the plot, the curve showing the slower decay of thisemission is approximately parallel to that for the emission of ˜1550 nmlight. The parallel nature of these curves is a consequence of theupconversion process, in which two ⁴I_(13/2) Er³⁺ ions (the ˜1550 muemitter) combine to form one ⁴I_(11/2) Er³⁺ ion (the ˜3000 nm emitter)and one ground state Er³⁺ ion. The rate of the cooperative upconversionis approximately proportional to the square of the ⁴I_(11/2) Er³⁺concentration, i.e., to the square of the ˜1550 nm emission intensity.Third, the ˜3000 nm emission is 4 to 5 times more intense from the REAl™glass than from the crystalline YAG material. Part of this difference isdue to a 50% greater Er concentration in the REAl™ glass. The remainingdifference in the intensities can be attributed to differences in (i)the Er ion absorption cross sections, (ii) the ⁴I_(13/2) upconversionrates, and (iii) the ⁴I_(13/2) radiant emission and quenching rates forthe two materials. The results show that the REAl™ glass materials areeffective sources of the ˜3000 nm radiation by comparison with the priorart Er-doped crystalline YAG material.

EXAMPLE 6 Glass Properties

Properties of the bulk glass materials were measured using standardtechniques. Density was measured by displacement using a 2 mlpycnometer, a microbalance and deionized water as the immersion fluid.Hardness was measured using a microhardness indenter. Glass transitionand crystallization temperature ranges were measured by differentialscanning calorimetry and differential thermal analysis. The dissolutionrate of the glass was investigated by immersing samples in agitateddeionized water at 363K (90° C.) and measuring the specific mass changeat intervals of 2 days over a period of 16 days. Index of refraction wasmeasured at wavelengths of 486, 589 and 659 nm (F, D and C Fraunhoferlines) using the Becke line method with index-matched oils. Abbe numberswere calculated from the measured refractive indices.

Table III presents properties of the REAl™ glass materials that havebeen measured on glasses formed either by levitation melting and coolingor by casting liquids melted in platinum crucibles.

The infrared transmission curves of 2 mm thick samples of two REAl™glasses containing no optically active dopants are shown in FIG. 10. Thefigure includes data from the literature for crystalline sapphire andpure silica glass of 2 mm thickness, for comparison purposes. Thetransmission curves for each material are: 11 silica, 12 REAl™ glasscontaining 20 mole % SiO₂, 13 REAl™ glass containing 5 mole % SiO₂, and14 sapphire. The figure illustrates that good transmission is obtainedat wavelengths beyond the infrared cut-off wavelength of silica glass.It is essential to minimize the silica content of glasses to obtain goodinfrared transmission in windows, lenses, and other optical elementsbeyond a wavelength of approximately 3 micrometers. This is possible inthe family of REAl™ glasses, which contain from zero to 30 mole % ofSiO₂.

Refractive index values measured for the REAl™ glasses are in the rangefrom 1.80 to 1.90, at the sodium D-line, 589 nm. Measurements at 486 and656 nm were also obtained to determine the Abbe numbers of the glasses.The Abbe numbers determined for REAl™ glasses are in the range fromapproximately 32 to approximately 66, depending on the glasscomposition. These properties are important in optical lenses, sincespherical aberration of the lenses is smaller for glasses with largervalues of the refractive index, and chromatic aberration of the lensesis smaller for glasses with larger values of the Abbe number. Thus,novel lenses can be fabricated from the REAl™ glasses with reducedchromatic and/or spherical aberration relative to lenses of similardesign that are fabricated from prior art materials.

Other modifications and alternative embodiments of the invention arecontemplated which do not depart from the scope of the invention asdefined by the foregoing teachings and appended claims. For example, thebulk single phase glass material used as the optical gain medium may besynthesized by any suitable method, including but not limited to themethods described herein and in commonly owned U.S. Pat. No. 6,482,758.Also, the gain medium may comprise well known optically active dopantsother than the ones described herein. The gain medium may also be pumpedby the application of light at wavelengths other than the ones describedherein and where at least one of the optically active dopant speciesabsorbs the light. It is intended that the claims cover all suchmodifications and alternative embodiments that fall within their scope.

1. An optical gain medium comprising a bulk single phase glasscomprising: (a) 27 to 50 molar % RE₂O₃; and (b) 50 to 73 molar % Al₂O₃;where RE is one or more elements selected from the group consisting ofSc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. 2.The optical gain medium of claim 1 used in a manner such that gain isgenerated by application of light in the wavelength range from 970-990nm.
 3. The optical gain medium of claim 1 wherein gain is generated byelectronic transitions of Yb.
 4. The optical gain medium of claim 1wherein gain is generated by electronic transitions of Er.
 5. Theoptical gain medium of claim 1 wherein gain is generated by electronictransitions of Tm.
 6. The optical gain medium of claim 1 wherein gain isgenerated by electronic transitions of Ho.
 7. An optical gain mediumconsisting essentially of a bulk single phase glass comprising one ormore rare earth oxides, aluminum oxide and silicon dioxide wherein thecomposition lies substantially within the heptagonal region of theternary composition diagram of the rare earth oxide-alumina-silicasystem defined by points having the following molar percentcompositions: 1% RE₂O₃, 59% Al₂O₃ and 40% SiO₂; 1% RE₂O₃, 71% Al₂O₃ and28% SiO₂; 23% RE₂O₃ and 77% Al₂O₃; 50% RE₂O₃ and 50% Al₂O₃; 50% RE₂O₃and 50% SiO₂; 33% RE₂O₃, 33.33% Al₂O₃ and 33.33% SiO₂; and 16.67% RE₂O₃,50% Al₂O₃ and 33.33% SiO₂.
 8. The optical gain medium of claim 7 used ina manner such that gain is generated by application of light in thewavelength range from 970-990 nm.
 9. The optical gain medium of claim 7wherein gain is generated by electronic transitions of Yb.
 10. Theoptical gain medium of claim 7 wherein gain is generated by electronictransitions of Er.
 11. The optical gain medium of claim 7 wherein gainis generated by electronic transitions of Tm.
 12. The optical gainmedium of claim 7 wherein gain is generated by electronic transitions ofHo.
 13. The optical gain medium of claim 1 wherein the REAl™ glass isformed by casting of a molten material.
 14. The optical gain medium ofclaim 7 wherein the REAl™ glass is formed by casting of a moltenmaterial.
 15. An optical material consisting essentially of a bulksingle phase glass comprising: (a) 27 to 50 molar % RE₂O₃; and (b) 50 to73 molar % Al₂O₃; where RE is one or more elements selected from thegroup consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb and Lu and wherein the glass is formed by casting of a moltenmaterial. 16 An optical material consisting essentially of a bulk singlephase glass comprising one or more rare earth oxides, aluminum oxide andsilicon dioxide wherein the composition lies substantially within theheptagonal region of the ternary composition diagram of the rare earthoxide-alumina-silica system defined by points having the following molarpercent compositions: 1% RE₂O₃, 59% Al₂O₃ and 40% SiO₂; 1% RE₂O₃, 71%Al₂O₃ and 28% SiO₂; 23% RE₂O₃ and 77% Al₂O₃; 50% RE₂O₃ and 50% Al₂O₃;50% RE₂O₃ and 50% SiO₂; 33.33% RE₂O₃, 33.33% Al₂O₃ and 33.33% SiO₂; and16.67% RE₂O₃, 50% Al₂O₃ and 33.33% SiO₂; where RE is one or moreelements selected from the group consisting of Sc, Y, La, Ce, Pr, Nd,Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu and wherein the glass isformed by casting of a molten material.